The use of a filament doping source for Si doping of molecular beam epitaxy (MBE) grown III–V semiconductors provides many benefits in the areas of cleanliness, simplicity of construction, and source lifetime. This work details the thermal and doping characteristics associated with such sources. Expressions are developed for the flux, temperature, and power relations, and for the heating and cooling characteristics. We show that the thermal response allows the doping to be varied by at least two orders of magnitude during the time taken to grow one monolayer at normal growth rates (1 μm/h), and for most applications this obviates the need for a shutter. The source is clean (largely because it is only the strip itself which is at an elevated temperature), repeatable, reliable, has a lifetime comparable with that of the system, and can achieve doping levels of 1019cm−3in GaAs with mobilities of 1300 cm2V−1 s−1at room temperature.